Dutch photographer Sarah Wong began the ongoing project Inside Out: Portraits of Cross-Gender Children in 2003, photographing children who have changed, or are in the process of changing, their sex because they feel they are a different gender to the one that they were born with.

"The Netherlands has probably the most progressive attitude toward gender variance in the world – transgender kids included," says Wong.

The children in these portraits were among the first to undergo a therapy developed by Peggy Cohen-Kettenis, the founder of Europe’s first clinic for children and adolescents with gender dysphoria, at VU University in Amsterdam.

According to Wong, from 20% of children with gender dysphoria, a carefully selected few will go on to take puberty blockers. At 16 they can proceed to cross-sex hormones, and at 18 surgery. The idea of the blockers is to delay puberty so the child gets time to explore his or her feelings. The effects are reversible; if they stop the drugs, their natural hormones will take over again. But the blockers’ long-term effects remain unknown.

"Initially I didn’t understand why the pictures were so important for the children, then I realised it’s because they show the person they feel they really are." – Sarah Wong

Every girl you see in these photos was born male, and every boy was born female.

Out of respect for the children's privacy we have not used their names. The quotes below are from both the children and their parents; the parents all use pronouns according to the sex their children were born with.

"His father gave him boys’ toys but he preferred draping himself in old curtains made to look like a dress. At 10 he became she, and started using the girls’ changing-rooms and toilets at school. She knows she can have an operation in the future but she’s not thinking about that yet." – Parent

14 years in 2007 (left) 17 years in 2010 with his boxing trainer (right)

"I think everyone should be open about it. Especially young kids in classrooms. That way you'll breed more empathy in schools and everyone will look out for each other. People will be less quick to pass judgment." – Child

"At home, her brother struggled to accept her. Gradually, things improved and she made good friends. When she started hormones and her breasts grew she was very excited ... Six months ago she had a sex change. 'Now,' she says, 'I can wear a bikini.'" – Parent

"It felt as if this was the way it was meant to be. Still, it was very painful. I gained a lovely daughter, but I lost a beautiful son… Life will never be easy for her. I worry that her heart will be broken." – Parent